The present invention relates to a magnetic disk drive including a magnetic disk medium that is rotationally driven and a magnetoresistive (MR) head that writes information onto or reads written information from magnetic disk media. More particularly, the invention is concerned with a magnetic disk drive that includes a function for preventing or alleviating trouble with the element section of an MR head due to the occurrence of thermal asperity on a magnetic disk.
The surface of a magnetic disk (hereinafter, may also be referred to simply as the disk) which is a recording medium of a magnetic disk drive is formed of a number of very thin layers stacked on one another from the disk substrate side in order, such as an underlayer, a magnetic layer, a protective layer, and a lubricant layer. The disk surface also has precise planarity to allow an MR head (hereinafter, may also be referred to simply as the head) to fly over the disk stably with a very small vertical clearance and conduct data read/write operations. Even so, projections equivalent to the very small vertical clearance, dents, flaws, or other defects are present on the disk surface.
When the MR head moves past over the projections on the disk, the element section of the head will come into contact with the projections if the height of the projections is greater than the flying height of the element section. At that time, since a relative speed exists between the MR head and the projections, frictional heat occurs and this causes an abrupt increase in the temperature of the MR head, and hence, an increase in the electrical resistance value of the MR element section. The direct-current components of a readout signal move as a result. This phenomenon is referred to as thermal asperity (TA), whereby a reading error will be caused.
In current magnetic disk drives, since the flying height of the head tends to decrease with increases in recording density, the clearance between the head and the disk is becoming narrower. Accordingly, projections of the height that has heretofore not become a problem are detected as TA-causing sections. There also occurs a situation in which, as the flying height of the head decreases, the very small dust and dirt whose presence has hitherto not become a problem get caught between the head and the disk, thus causing TA. For these reasons, it is difficult to prevent TA from occurring. Assembly of such magnetic disks into a drive is usually followed by the adoption of a method in which the disks are formatted, except for defective sections, and then all defective sections are registered in the drive prior to the start of its use under normal conditions in order to prohibit accessing these defective sections.
Patent Document 1 (Japanese Patent Laid-Open No. Hei 10-3603) discloses a method of detecting and registering the defective sections of a disk medium that cause TA. The detection of the defective sections that cause TA is accomplished by detecting changes in the thermal resistance of the MR element section of an MR head. During the registration of the detective sections, the same sectors of several tracks adjacent to these TA-causing sections are registered as defective sectors to prevent the occurrence of a readout error due to subsequent TA.
Patent Document 2 (Japanese Patent Laid-Open No. 2000-293946) describes a method of reducing the size of a defect information list preservation region significantly and allowing a skipping process to be performed within a short time. In this method, cylinders to which the defective tracks that have suffered damage to become defective sections at which normal data writing or reading is impossible belong are registered in a defective-cylinder list beforehand and when an access request is received from a host apparatus, the defective cylinders registered in the defective-cylinder list are skipped and only nondefective cylinders not including defective tracks are used.